Btrfs: send, add missing check for dead clone root

After we locked the root's root item, a concurrent snapshot deletion
call might have set the dead flag on it. So check if the dead flag
is set and abort if it is, just like we do for the parent root.
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/send.c

@@ -5855,7 +5855,8 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 			clone_sources_to_rollback = i + 1;
 			spin_lock(&clone_root->root_item_lock);
 			clone_root->send_in_progress++;
-			if (!btrfs_root_readonly(clone_root)) {
+			if (!btrfs_root_readonly(clone_root) ||
+			    btrfs_root_dead(clone_root)) {
 				spin_unlock(&clone_root->root_item_lock);
 				srcu_read_unlock(&fs_info->subvol_srcu, index);
 				ret = -EPERM;